1. Field of the Invention
Embodiments of the invention relate to an organic light emitting diode (OLED) display device, and more particularly, to a top emission type OLED display device having an improved display quality.
2. Discussion of the Related Art
FIG. 1 shows the related art OLED display device.
The OLED display device as a flat panel display device is a self-emission type display device. The OLED display device includes a first electrode 10, a second electrode 70 and an organic layer 60 therebetween. Voltages are applied to the first and second electrodes 10 and 70, a current is supplied to the organic layer 60 such that an emitting layer of the organic layer 60 emits light. The emission from the organic layer 60 can be controlled by controlling the voltages or the current.
The OLED display device may be classified into a passive matrix type and an active matrix type according to a driving method. In addition, the OLED display device may be classified into a top emission type and a bottom emission type according to an image displaying surface. In the top emission type OLED display device, the image is displayed on a front surface of a substrate. In the bottom emission type OLED display device, the image is displayed on a bottom surface of the substrate.
In the active matrix type OLED display device, a switching element 1 is positioned in each pixel such that each pixel is controlled by the switching element 1. The switching element 1 may include a thin film transistor and a capacitor and receive a signal and a power to control the organic emitting diode. The switching element 1 controls the voltage or the current, which is supplied to the organic emitting diode, and the first electrode 10 of the organic emitting diode is connected to the switching element 1 to receive the voltage or the current.
The switching element 1 is covered with a passivation layer 80, which includes an organic material or an inorganic material, and the first electrode 10 is electrically connected to the switching element 1 through a contact hole in the passivation layer 80. The first electrode 10 is formed in each pixel, and the first electrodes 10 in adjacent pixels are separated by a pixel definition layer 30. A portion of the first electrode 10, which is not covered with the pixel definition layer 30, is defined as an emitting region.
The active matrix type OLED display device requires an area for the switching element 1. However, since the top emission type OLED display device displays images on a front surface, the area for the switching element 1 is also used as a display area by disposing the organic emitting diode over the switching element 1. The first electrode 10 is disposed over the switching element 1 to overlap the switching element 1, and the organic layer 60 including the emitting layer and the second electrode 70 is formed on the first electrode 10. Since the light from the emitting layer passes through the second electrode 70 in the top emission type OLED display device, the second electrode 70 is formed of a transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO).
The second electrode 70 of the OLED display device is connected to a ground voltage or a reference voltage of a constant voltage. A reference power is connected to the second electrode 70 through a pad at a peripheral region of the substrate, and the reference power is supplied to each pixel through the second electrode 70. In the related art OLED display device, the current in each pixel is discharged into an outer reference voltage through the second electrode 70.